1. Field of the Invention
The invention relates generally to drilling and mining equipment, and more specifically to generators for electrically powering telemetry and radar instrumentation at the distal end of a drillstring using the flow of hydraulic fluids supplied to the drill bits.
2. Description of the Prior Art
Instrument packages placed at the ends of drillstrings can provide important information from radar scans to help guide direction drilling efforts. Both the radar and the telemetry used to communicate data and commands need a safe source of electrical power. Typical drillstrings can be miles long and under great pressure from the fluids and gases being pumped around and through them, and the depths of earth explored. Wiring power through the drillstrings and into explosive methane and coal deposits is not practical. So what is needed is an intrinsically safe power generator that can provide enough power in the severe environments encountered.
The production of coal and methane depends upon the environment of the original coal bed deposit, and any subsequent alterations. During burial of the peat-coal swamp, sedimentation formed the sealing mudstone/shale layer overlying the coal bed. In deltaic deposits, high-energy paleochannels meandered from the main river channel. Oftentimes, the channels scoured through the sealing layer and into the coal seam.
High porosity sandstone channels often fill with water. Under the paleochannel scour cut bank, water flows into the face and butt cleats of the coal bed. Subsequent alterations of the seam by differential compaction cause the dip, called a roll, to occur in the coal bed. Faults are pathways for water flow into the coal bed.
Drilling into the coal bed underlying a paleochannel and subsequent fracturing can enable significant flows of water to enter. The current state of the art in horizontal drilling uses gamma sensors in a measurements-while-drilling (MWD) navigation subsystem to determine when the drill approaches a sedimentary boundary rock. But if sandstone is protruding into the coal, such as results from ancient river bed cutting and filling, then the gamma sensor will not help. Sandstone does not have significant gamma emissions, so this type of detection is unreliable. Drilling within the seam cannot be maintained when the seam is not bounded by sealing rock.
Methane diffusion into a de-gas hole improves whenever the drillhole keeps to the vertical center of the coal seam. It also improves when the drillhole is near a dry paleochannel. Current horizontal drilling technology can be improved by geologic sensing and controlling of the drilling horizon in a coal seam.
There are a number of conventional ways directional drills use to steer in a desired direction. One involves placing the drill bit and its downhole motor at a slight offset angle from the main drillstring. The whole drillstring is then rotated to point the offset angle of the drill bit in the direction the operator wants the borehole to head. Another method involves an articulated joint or gimbal behind the drill bit and its downhole motor and using servo motors to angle the joint for the desired direction.